1. Field of the Invention
The field of the invention is that of plasma reactors.
The invention can be applied notably to the surface treatment of samples, for example the etching of a layer of silicon oxide deposited on a micro-electronic substrate. More generally, the plasma reactor of the invention concerns the treatment of objects of any type including, possibly, fragile materials such as polymers, irrespectively of their size and number. It can be used, in particular, to obtain large-sized plasmas.
2. Description of the Prior Art
There already exist known plasma reactors enabling a surface treatment of samples. Such reactors may notably include an enclosure capable of receiving a chosen gas flow, and a generator of high-frequency electromagnetic waves. The plasma is obtained by placing the gas flow in the presence of the electromagnetic waves.
The reactor therefore also includes means for coupling the enclosure with the electromagnetic wave generator. To be efficient, this coupling should be non-resonant with the enclosure. In other words, the enclosure should not dissipate, in the form of electromagnetic radiation, the high-frequency energy communicated to it in the presence of gas.
In one particular approach to providing for a non-resonant coupling with a large-sized enclosure, the end part of the coupling means comprises a horn that is gradually thinned in one direction and widened in the other direction until a flat rectangular cross-section completely surrounding said enclosure is achieved.
To provide notably for a uniform distribution of the electromagnetic energy around the chamber, in order to contribute to a satisfactory homogeneity of the plasma, the horns used inside these coupling means should provide high gain while, at the same time, minimizing the modification of the phase of the electrical field between the input and the output of the horn.
Now, the phase modifying factor depends, as a rough estimate, on a function of the square of the aperture of the horn. This has earned it the designation, commonly used by those skilled in the art, of "phase r.m.s. error".
It would therefore appear to be necessary to restrict the angle of aperture of the horn as much as possible. This then also makes it possible to consider the components of the higher modes of the electrical field as being negligible as compared with those of the dominant mode.
As a consequence, obtaining a high gain then dictates the increasing of the length of the horn.
However, the space taken up by such reactors, which is directly related to the length of the horns, proves to be a penalizing factor in certain applications which require, notably, the use of plasma reactors of large sizes or of any shapes.
For, as a rule, the plasma reactors are placed in clean room environments, the cost of which is very high and essentially related to the volume of the rooms. It would therefore appear to be very important to reduce the space occupied by such reactors.
To make the plasma homogeneous, in order to obtain more efficient surface treatment of samples for example, provision may also be made to apply a magnetic field of diffusion in the enclosure.
The application of such a magnetic field has the drawback of using large quantity of energy, sometimes greater than that used to produce the plasma.
The result thereof is a restriction of the dimensions of the reactor and, consequently, a restriction of the dimensions of the plasma obtained. This prevents the treatment of large samples.
Besides, for certain types of processing, the plasma obtained in such reactors proves to have insufficient homogeneity.
It is an aim of the invention to mitigate these various drawbacks.